edit: I managed to sweep using electrum desktop, all I had to do was add p2wpkh: in front of the private key.
On Android, I prefer Mycelium for watch-only wallets. When importing, it automatically checks all addresses associated with a private key (so no manual "p2wpkh:" needed, I can never remember that)).
still looking into why it doesn't work on android, with samourai or bitcoin core...
I've had inconsistencies with watch-only wallets on Bitcoin Core too. I can't find back my post about it though.